Responses of riverbed morphology and benthic invertebrates to varying quantities of sediment supplied through bypass tunnel downstream of Koshibu Dam, Japan

Riverbed and ecosystem responses to sediment supply can vary from superficial to profound, depending on the quantity and quality of the sediment. Substantial alterations in cross-sectional morphology, habitat structure, and benthic invertebrate communities were observed downstream of the Koshibu Dam during periods of high sediment transport (0.4–1.8 million m³/year) through the sediment bypass tunnel (SBT). The channel widened by bank erosion in the confined reaches and narrowed by deposition in the unconfined reaches, with an extent of 20–40 m. Bed grain size decreased, while Shield’s parameter increased across the reaches. As a consequence of increased fast-flow habitats (e.g., rapids), the species composition of benthic invertebrates shifted toward a more natural state, as indicated by the Bray–Curtis similarity index. Despite these considerable changes, certain habitat features and invertebrate assemblages returned to their previous states during a two-year suspension of SBT operation. These results demonstrate the importance of the quantity of sediment supply, the characteristics of the sediment-receiving channel, and the continuity of sediment supply for ecosystem restoration through sediment management.